Quote:
Originally Posted by Brandon002
So you're one of those people that thinks changing the fluid on a high mileage trans will have negative effects?
I'd love to hear the reasoning behind this one.
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It is a failure mode that is fairly well understood. But is it not that there will be negative effects. There may be negative effects, in a limited time window. There is a difference. We are speaking probabilities here, not absolutes.
Transmission fluid, by design, has a high level of detergents. That is so that it lasts a long time. Over an extended time, those detergents become less effective. Varnish and deposits can start to build up in the transmission, in places where they are not doing any particular harm. Then, the fluid is changed, and those strong new detergents scour the internals of the transmission. The crud that is released and moved can make it to the filter. If so, fine, no problem. But if it first gets lodged in the valve body, in one of the orfices or check valves or actuators, it can precipitate a failure. There is a theory that this can also happen when the transmission is first filled, and started, and the fluid courses through, but I think the detergents matter more, as failures when they happen usually aren't on start up but in the weeks that follow.
There is no such thing as shocking the fluid. There is simply cleaning out a system that isn't completely clean, and not having any place for the residue to go. Except through a valve body with a lot of very small passageways.
If there is no failure within a few thousand miles (purely an estimate, no science here) I would say that a subsequent failure, if it occurs, isn't related to the fluid change. But if a transmission is working perfectly, and the fluid is changed at a higher mileage, and the transmission fails soon after, it is a recognized issue. We are not speaking here about the wrong fluid, or doing it wrong. And not about transmissions that already had a problem. In those cases, changing the fluid is a valid attempt to address the problem, and subsequent failures can not be blamed on the fluid change.
The above failure mode is well enough known and understood that many shops will decline to change the fluid on a high mileage transmission that hasn't had a previous fluid change. They don't want to take on the risk. Because if the transmission fails, the owner will blame the shop and demand compensation. It is often better for their bottom line to just turn down those jobs and let someone else take the bullet. This has nothing to do with believing what the manufacturer says or not about changing the fluid. It is simply a risk/reward calculation.
The way to reduce the above risk is to change the fluid frequently enough that crud doesn't build up at all. Some use the interval of 30,000 or 50,000 miles. I think that is a valid approach. It is just that if you are going to do four or five or more fluid changes before the transmission fails, you should be pretty sure that you will get a return on your investment. One change is pretty cheap. Four or five, not so much. And recall that if the transmission fails due to an electronic component like a sensor or wiring harness, as many do, or a random break not impacted by lubrication, then those multiple fluid changes will have been for naught in the economic analysis of return on investment.